This invention relates generally to pelletizing of particulate matter and more particularly to preconditioning of animal feed materials before pelletizing.
Prior to pelletizing, animal feeds must be finely ground and exposed to a conditioning treatment. This involves heating in an environment with controlled moisture content to cause a desired degree of starch conversion and to destroy salmonella organisms in the feed. In addition, this conditioning treatment, by increasing moisture content and temperature of the feed, improves the pellet formation ability of the feed in the pelletizing die. Conditioning is normally performed in the mixer in which the feed formulation is blended. The conditioner is heated and moisture is added by means of a steam valve which is throttled to control the temperature and moisture content of the feed formulation.
A conventional feed pelletizing mill includes a mixer/conditioner, a transfer unit to convey the conditioned material to the pelletizing die, the pelletizing die, and a pellet cooling unit. The conditioner size is determined by the dwell time required by the feed material in the conditioner. Thus, material requiring a long conditioning dwell time will require a large conditioner in order to provide a adequate supply of feed material to the pelletizing die. Dwell time required is determined by the characteristics of the feed material, such as particle size, permeability to heat and moisture, heat capacity, starch content, and shear strength, and by the time, temperature and moisture required to kill microorganisms. In general, the finer the particle size, the shorter the conditioning time required.
Ideally, conditioning would be performed under temperature and moisture conditions which are optimized for the material being conditioned. This is not always possible because, if the temperature and moisture content required for conditioning o the material are greater than that which can be tolerated in the pelletizing die due to the shear strength of the feed material the conditioning parameters must be adjusted in order to avoid plugging of the pelletizing die. It is generally accepted that the shear strength of the feed material is inversely proportional to its temperature and moisture content. Thus, when temperature and moisture content become too high, the shear strength decreases to the point where the feed material plasticizes and begins to agglomerate at pressures below that necessary for pelletizing, thereby plugging the pelletizing die.
It would be desirable to avoid the cost of fine grinding of the feed materials and to only grind them sufficiently to provide proper mixing, however, the constraints imposed by the long conditioner dwell time required by coarse particles creates risks which outweigh the economic advantage of coarse grinding. Since large particles require long conditioner dwell times, a pelletizing die of a given capacity will require a larger conditioning unit. Larger conditioners not only have a higher cost, but also create operational problems. If the steam valve control is not accurate, the feed material exiting the conditioner may be overheated and contain excess moisture, thus, it may have too low a shear strength for proper pelletizing. When that happens the conditioner must be emptied and purged in order to reduce its temperature and moisture content. This requires shutdown of the pelletizing operation until the conditioner is completely emptied and purged. Such shutdowns are costly.
The foregoing illustrates limitations known to exist in present pelletizing operations. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.